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<head>
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    <title>Document</title>
    <style>
        .container{
            width: 80%;
            margin: 0 auto;
            border: 1px solid red;
        }

        /* 当屏幕尺寸大于等于700px时 */
        @media screen and (min-width: 700px){
            .container{
                font-size: 50px;
            }
        }

        /* 当屏幕尺寸小于等于400px时 */
        @media screen and (max-width: 400px){
            .container{
                font-size: 20px;
            }
        }

        /* 当屏幕尺寸大于等于401px时，并且小于等于699px时 */
        @media screen and (min-width: 401px) and (max-width: 699px) {
            .container{
                font-size: 40px;
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    </style>
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<body>
    <div class="container">
        跟汤哥学开发
    </div>
</body>
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